Did you do anything that you think may have pissed Google off? If not, you can apply for reconsideration to get your site re-indexed. Have you read their guidelines?
How big was it? If its a new website, its normal. Google tends to rank new websites high for a while, and then slowly drop it when it gets no new content or imbound links from stable websites. Maybe you also lost some links or something? Or did u piss Google off with something?
How can you say, you site was deindexed ? your site might sandboxed by Google. Check with Google Data centers or Contact by mail to google administrator. If you did anything wrong then fix the problem and re submit you site into google.
I have had the same problem, got indexed within a couple of days, then my home page got de indexed. I was only building links the way I have read, social bookmarking, articles, forums & blog commenting. What did I do wrong? I never made any more than 10/15 links a day.
Goto webmaster tools from google and tell them that you correct the site (you need to correct it first).
I think the reason of it is having bad backlinks or writing duplicate content for your website. I suggest you to check above points. If the reason is your backlinks, you should stop making backlinks for your website from low quality pages and making some backlinks for it from high quality pages slowly. And if the reason is content of your website, you should write high quality unique contents for your website and then make some backlinks for these new pages from quality websites. Also its better that you sign up for Google webmaster tools and following the statistics of your website using it. Also making a XML sitemap and submitting in in Google webmaster tools, can help your website.
Just Do some Social Bookmarking Bookmarking for all your page in Top 10 Bookmarking sites.it's really help u to index your all page Quickly as possible.
If it is done by Google and you have not made any changes recently in your website then I think you are penalized by Google. NOw you have to write rich original contents and bookmark them. Work on getting backlinks also.